Liquid crystal displays use liquid crystal compositions for display. The liquid crystal displays are used as display devices for various machines, in particular, as information display devices and image display devices.
Such a liquid crystal display shows an image by transmitting or blocking light on a region basis, according to voltage application. Accordingly, to show an image on the liquid crystal display, external light is needed. As a light source for the external light, a backlight is provided for use on a back surface of the liquid crystal display. Conventionally, cold cathode tubes are used for such backlights. Recently, there are circumstances where LEDs (light emitting diodes) are used instead of cold cathode tubes, for the reasons of long life, good color development, and the like.
In recent years, nanosized phosphors using quantum dots have been commercialized mainly by venture companies overseas. Quantum dots are luminescent semiconductor nanoparticles and have a diameter in a range of 1 to 20 nm. The unique optical properties and electronic properties of the quantum dots are being utilized in many applications, such as flat panel displays and illumination (decorative lighting) with a wide variety of colors, in addition to fluorescent imaging applications in the fields of biology and medical diagnosis.
White LED technology, which plays a supremely important role in displays, generally uses a method of exciting cerium-doped YAG-Ce (yttrium-aluminum-garnet) phosphors for down conversion, by means of a blue (450 nm) LED chip. When the blue light of the LED becomes admixed with the yellow light generated from the YAG phosphors with a wide wavelength range, white light is created. However, this white light commonly is somewhat bluish and therefore is taken to be a cold white or cool white.
The quantum dots exhibit a wide excitation spectrum and have high quantum efficiency and thus can be used as LED down conversion phosphors. The quantum dots can have an emission wavelength fully adjusted over the entire visible region by only altering the dot size or the type of the semiconductor material. Therefore, the quantum dots can create substantially any color, in particular, warm whites that are strongly desired in the illumination industry. In addition, combinations of three types of dots having emission wavelengths corresponding to red, green, and blue enable white lights having different color rendering indices. Thus, displays provided with a backlight unit using phosphors composed of quantum dots can improve hue and can express up to 65% of the colors that can be distinguished by a person, without increasing the thickness, power consumption, costs, or manufacturing processes more than those of conventional liquid crystal TVs.
Such a backlight unit is configured by combining a wavelength conversion sheet with an LED light source and a light guide plate. The wavelength conversion sheet is obtained by diffusing quantum dots having an emission spectrum of red or green into a film, and sealing a surface thereof, and its edge portions as well in some cases, with a barrier film or a lamination of barrier films. The barrier film is required to have an appearance without wear and tear, wrinkles or the like, and to have transparency, in addition to barrier properties. However, conventional barrier films cannot provide satisfactory performance since they have only been used as packaging materials for foods, medical supplies, and the like, or packaging materials for electronic devices and the like.
Several methods have been conceived in order to solve such problems. For example, PTL 1 proposes a configuration sandwiching a layer having phosphors between barrier films to minimize degradation of the phosphors. In addition, PTL 2 proposes coating a device with a barrier film to ensure reliability of an organic EL device.
With reference to PTLs 1 and 2, a display was prepared by sealing quantum dots with an existing barrier film. However, the obtained white light had a short life due to the insufficient barrier properties, and had unevenness in the white LED light due to wear and tear, and wrinkles in the film, and due to patterns of the quantum dots, and the like.